Abstract: A method of screening for a protein involved in the extracellular regulation of latent TGF? activation by transducing a cell line with a retroviral cDNA library to create a reporter cell line that produces green fluorescent protein (GFP) in response to TGF-? signaling; growing individual clones created by the reporter cell line; co-culturing each individual clone with a second TGF-? reporter cell line that produces luciferase in response to TGF-?, wherein the luciferase production identifies positive clones; and identifying a mechanism of latent TGF-? activation that is employed by the positive clones. A TGF? reporter cell line including a cell line; and a retroviral cDNA library, wherein the reporter cell line produces GFP in response to TGF-? signaling. A method of screening for gene products involved in a biological process.